The present invention relates to a liquid crystal display device and a method of manufacturing a backlighting light guide panel for the same.
In recent years, implementation of personal computers inclusive of so-called word processors in a small size has been promoted, and portable type personal computers known as lap-top type or notebook type computers are widely used. In such portable type personal computer, a liquid crystal device is commonly used as a display unit. In this conjunction, there is an increasing tendency for adopting color display in the portable type personal computers. In accompanying with such trend, a backlighting type display device is coming into wide use, in which a light source is disposed at a rear side of a liquid crystal display screen for lighting the whole display screen from the rear or back side. Needless to say, the backlighting light source for the color liquid crystal display device is required to emit light with high luminance. Besides, it is necessary to illuminate the display screen with uniform luminance over the whole planar surface thereof. Luminance of the backlighting can easily be increased by increasing that of the light source. However, taking into consideration the fact that the portable-type personal computer or word processor or the like are usually operated by using a battery or cell, limitation is necessarily imposed to the attempt for increasing the luminance of the light source. To say in another way, there has been proposed no effective method or measures for increasing the luminance of the liquid crystal display screen.
For having better understanding of the present invention, description will first be made in some detail of conventional liquid crystal display devices such as disclosed, for example, in JP-A-4-162002 and JP-A-6-67004. FIG. 3 shows a lateral source type backlighting device employed conventionally in the liquid crystal display device known heretofore. Referring to the figure, a lamp such as a cold-cathode discharge tube or a hot-cathode discharge tube is employed as a light source 1 which is disposed at and along one lateral side of a light guide plate (also known as optical waveguide plate) 2 which is made of a light-transmissive material, wherein a diffusing sheet 3 formed of a synthetic resin of milk-white color having a light scattering effect is mounted over a top surface of the light guide plate 2 with a view to uniformizing luminance of the backlight over the whole display screen. Additionally, there are disposed on the diffusing sheet 3 a first prism sheet 4 and a second prism sheet 5 for the purpose of enhancing axial luminance (luminance in the direction orthogonal to the display screen) of the display device by converging diffused light rays.
In addition, a light scattering layer 6 is deposited over a rear surface of the light guide plate 2 at a side opposite to the light exit side in order to scatter the light rays traveling through the light guide plate 2 in the direction toward the diffusing sheet 3. In this conjunction, the light scattering layer 6 is manufactured in a specific structure described below with the aim to further uniformize luminance distribution of the light rays exiting the light scattering layer 6.
FIG. 4 of the accompanying drawing shows a structure of the light scattering layer 6. As can be seen in this figure, the light scattering layer 6 is formed by a plurality of light scattering dots by depositing titanium oxide or the like over the rear surface of the light guide plate 2 by resorting to e.g. a printing technique. Parenthetically, it will readily be understood that the intensity of light emitted from the light source 1 becomes lower as the distance from the light source 1 increases. Accordingly, the light scattering dots of the light scattering layer 6 deposited over the bottom surface of the light guide plate 2 are so formed that the area of a given dot increases as the position of the given dot becomes more remote from the light source 1. Furthermore, a reflecting sheet 8 is disposed on a bottom surface of the light scattering layer 6, as can be seen in FIG. 3.
According to another proposal disclosed, for example, in JP-A-7-294745, grating grooves are formed in the bottom surface of the light guide.
As is apparent from the foregoing description, in the conventional backlighting devices for the liquid crystal display device known heretofore, light emitted from the light source 1 and introduced into the optical waveguide or light guide plate 2 undergoes scattering at the light scattering dots forming the light scattering layer 6 so that the scattered light rays can be reflected back again into the light guide plate 2 under the action of the reflecting sheet 8 to thereby illuminate the liquid crystal cell after transmission through the diffusing sheet 3 and the two prism sheets 4 and 5. It can readily be understood that the structure of the conventional backlighting optical waveguide or light guide panel for the liquid crystal display device is much complicated.
Besides, because the diffusing sheet 3 of a light absorbing material is disposed over the top surface of the light guide plate 2, the conventional liquid crystal display device suffers a drawback that the luminance of the liquid crystal display device becomes lower as a whole although nonuniform distribution of luminance can certainly be suppressed to some extent. In other words, with the structure of the conventional backlighting device, the attempt for increasing the luminance is incompatible with the attempt for uniformization of luminance distribution. To say in another way, it is impossible to meet simultaneously both requirements for increasing the luminance on one hand and for uniformizing the luminance distribution on the other hand. Furthermore, in the case of the conventional liquid crystal display device in which the grating grooves are provided in the light guide, the pattern of the grating grooves reflected to the light rays exiting the light guide panel will interfere with a regular pattern of elements such as that of liquid crystal cell constituting the liquid crystal display device, giving rise to a problem that moire phenomenon makes appearance. In order to solve this problem, a sheet for diffusing the light rays has to be additionally provided, to another disadvantage.
Furthermore, with the structure of the conventional backlighting optical waveguide or light guide panel, difficulty is encountered in mounting fixedly and stationarily the reflecting sheet 8 because of difference in the thermal expansion coefficient between the light guide plate 2 and the reflecting sheet 8 due to heat transfer from the light source 1, resulting in variation in the distance between the reflecting sheet 8 and the rear or bottom surface of the light guide plate 2 due to vibration, thermal deformation or the like phenomena, which in turn brings about a problem that nonuniformness of luminance distribution is likely to occur due to variation in the light utilization efficiency, as brought about by invasion and deposition of dusts between the reflecting sheet 8 and the light guide plate 2.